Various types of sensors exhibiting magnetoresistive characteristics are known and implemented in systems, particularly for reading of information signals recorded in magnetic media such as tapes, drums and diskettes. These sensors typically comprise a block made of a ferromagnetic alloy exhibiting high magnetoresistance. A recording medium passing in close proximity to such a sensor causes variations in the magnetic field at the point of the read head, and hence variations of the electrical resistance of the magnetoresistive sensor.
Recently however, magnetoresistive sensors have been described exhibiting a form of magnetoresistance generally known as "spin-valve"(SV) magnetoresistance, in which the change in resistance of the sensor is attributed to the spin-dependent transmission of conduction electrons between the magnetic layers of the sensor and the accompanying spin-dependent scattering at the layer interfaces. In such a sensor, the magnetoresistance is observed to vary as the cosine of the angle between the magnetizations of the layers and is dependent upon the deviation of current flow through the sensor. While such sensors exhibit a magnetoresistance that, for selected combinations of materials, is greater in magnitude than that exhibited by anisotropic magnetoresistors (AMR), they suffer from the relatively small magnitudes of the magnetoresistance at ambient temperatures.
Most recently, magnetoresistance sensors for reading/writing information signals stored on a magnetic medium have been described in issued patents and patent applications Ser. No. 08/396,819, filed Mar. 2, 1995 now abandoned; U.S. Pat. No. 5,646,051, issued Jul. 8, 1997; U.S. Pat. No. 5,696,655, issued Dec. 9, 1997; Ser. No. 08/781,994, filed Jan. 6, 1997 now pending and Ser. No. 08/917,058, filed Aug. 22, 1997 now pending, each assigned to the same assignee as the instant application and incorporated herein by reference.
The prior art also describes non-magnetic giant magnetoresistive sensors constructed from a thin film of non-homogeneous semiconducting magnetoresistive material, e.g. mercury cadmium telluride.
It is often assumed that microscopic inhomogeneities in semiconductors cause a diminution of the carrier mobility due to the additional scattering associated with the inhomogeneity. However, following the seminal work of C. Herring, entitled "Effect of Random Inhomogeneities on Electrical and Galvanomagnetic Measurements", in Journal of Applied Physics, Vol. 31, No. 11, pps. 1939-1953 (1960), C. M. Wolfe et a, in an article entitled "High Apparent Mobility in Inhomogeneous Semiconductors" in Solid State Science and Technology, Vol. 119, No. 2, pps. 250-255 (1972) showed that conducting inhomogeneities in semiconductors could actually result in a huge increase in the "apparent" Hall mobility relative to the actual carrier mobility by as much as a factor of 10.sup.3. The present invention shows that the same physical effect which boosts the Hall mobility also boosts the apparent giant magnetoresistance (GMR), and that this GMR boost may have important consequences for magnetic sensor technology, especially for high mobility semiconductor read-heads in high density magnetic recording.
One such high mobility semiconductor is mercury-cadmium-telluride (MCT) which has the alloy composition Hg.sub.1-x Cd.sub.x Te, 0&lt;x&lt;1. The alloy with composition x.about.0.22 has been extensively employed as a radiation emitter and detector compound in optical devices operating in the 10 .mu.m spectral region. Recently, however, Solin et al., in an article entitled "Self-biasing nonmagnetic giant magnetoresistance sensor", in Applied Physics Letter, 69 (26), pps. 4105-4107 (1996) have shown that the thin film MCT with a composition of x.about.0.1, corresponding to a (near) zero band-gap, exhibited a CGMR (measured using the Corbino disc geometry) which made it competitive with., if not superior to, more conventional metallic GMR detectors such as spin-valves as described by B. Dieny et al, in an article entitled "Magnetotransport properties of magnetically soft spin-valve structures", in Journal Applied Physics, 69 (8), pps. 4774-4776 (1991) and B. Dieny et al, in an article entitled "Giant magnetoresistance in soft ferromagnetic multilayers", Physical Review B, Vol. 43, No. 1, pps. 1297-1300, (1991). This superiority was in part a consequence of the enhancement of the low field GMR (.mu.H&lt;1 where .mu. is the carrier mobility) by a factor of 30 or more over that which was expected on the basis of the high field GMR as noted in Solin et al. Indeed, this low field enhancement had been observed in bulk material many years ago by Korol' et al., in an article entitled "Magnetoresistance of CdHgTe near gapless state", in Sov. Phys. Semicond., Vol. 11, No. 3, pps. 288-289 (1977) and by Korol' et al., in an article entitled "Investigation of Cd.sub.x Hg.sub.1-x Te magnetoresistors in the temperature range 4.2-300.degree. K.", in Sov. Phys. Semicond., 12 (3), pps 275-277 (1978) but was not appreciated or explained. Solin et al supra realized the technological significance of the enhanced GMR and used a phenomenological model to describe it, but acknowledged a lack of understanding of the physics underlying the enhancement.